The present invention relates to electrical filters and in particular to power supply filters.
Most modern electrical circuits are powered by Direct Current (DC) power supplies that ideally produce a constant voltage and as much current as a circuit requires. Typically, a DC power supply's voltage and current are either taken directly from the terminals of a battery or from a rectifier that has rectified an alternating current (AC) power signal. Although these power sources should ideally provide noise-free voltages and currents, their outputs often include noise frequencies. This noise is undesirable because it can appear in the output signal of the circuit powered by the power supply.
To eliminate these unwanted noise frequencies, additional filters have been used that screen out frequencies above zero hertz. These filters appear between the DC power source and the remainder of the circuit. Ideally, such filters should not change the DC voltage or the DC current provided to the circuit. However, in practice, most filters introduce a voltage drop between the power source and the circuit or limit the amount of current available to the circuit.
For instance, many prior art filters include an output stage that has a transistor configured as an emitter-follower. This transistor provides the current needed to drive the attached circuit. Unfortunately, emitterfollowers have an inherent voltage drop from their bases to their emitters and this voltage drop lowers the voltage provided to the circuit. Although removing the emitter-follower would eliminate this voltage drop, the resulting filter would not provide enough current to power the circuit.
In light of the state of the prior art, an efficient power supply filter is needed that provides sufficient current while eliminating the voltage drop found in an emitter-follower output stage.